[PATCH] uml: implement soft interrupts

This patch implements soft interrupts.  Interrupt enabling and disabling no
longer map to sigprocmask.  Rather, a flag is set indicating whether
interrupts may be handled.  If a signal comes in and interrupts are marked as
OK, then it is handled normally.  If interrupts are marked as off, then the
signal handler simply returns after noting that a signal needs handling.  When
interrupts are enabled later on, this pending signals flag is checked, and the
IRQ handlers are called at that point.

The point of this is to reduce the cost of local_irq_save et al, since they
are very much more common than the signals that they are enabling and
disabling.  Soft interrupts produce a speed-up of ~25% on a kernel build.

Subtleties -

    UML uses sigsetjmp/siglongjmp to switch contexts.  sigsetjmp has been
    wrapped in a save_flags-like macro which remembers the interrupt state at
    setjmp time, and restores it when it is longjmp-ed back to.

    The enable_signals function has to loop because the IRQ handler
    disables interrupts before returning.  enable_signals has to return with
    signals enabled, and signals may come in between the disabling and the
    return to enable_signals.  So, it loops for as long as there are pending
    signals, ensuring that signals are enabled when it finally returns, and
    that there are no pending signals that need to be dealt with.

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/um/include/longjmp.h

@@ -0,0 +1,19 @@
+#ifndef __UML_LONGJMP_H
+#define __UML_LONGJMP_H
+
+#include <setjmp.h>
+#include "os.h"
+
+#define UML_SIGLONGJMP(buf, val) do { \
+	siglongjmp(*buf, val);		\
+} while(0)
+
+#define UML_SIGSETJMP(buf, enable) ({ \
+	int n; \
+	enable = get_signals(); \
+	n = sigsetjmp(*buf, 1); \
+	if(n != 0) \
+		set_signals(enable); \
+	n; })
+
+#endif

arch/um/os-Linux/main.c

@@ -81,20 +81,8 @@ extern void scan_elf_aux( char **envp);
 int main(int argc, char **argv, char **envp)
 {
 	char **new_argv;
-	sigset_t mask;
 	int ret, i, err;
 
-	/* Enable all signals except SIGIO - in some environments, we can
-	 * enter with some signals blocked
-	 */
-
-	sigemptyset(&mask);
-	sigaddset(&mask, SIGIO);
-	if(sigprocmask(SIG_SETMASK, &mask, NULL) < 0){
-		perror("sigprocmask");
-		exit(1);
-	}
-
 #ifdef UML_CONFIG_CMDLINE_ON_HOST
 	/* Allocate memory for thread command lines */
 	if(argc < 2 || strlen(argv[1]) < THREAD_NAME_LEN - 1){

arch/um/os-Linux/process.c

@@ -18,6 +18,7 @@
 #include "process.h"
 #include "irq_user.h"
 #include "kern_util.h"
+#include "longjmp.h"
 
 #define ARBITRARY_ADDR -1
 #define FAILURE_PID    -1
@@ -205,24 +206,13 @@ void init_new_thread_signals(int altstack)
 
 int run_kernel_thread(int (*fn)(void *), void *arg, void **jmp_ptr)
 {
-       sigjmp_buf buf;
-       int n;
-
-       *jmp_ptr = &buf;
-       n = sigsetjmp(buf, 1);
-       if(n != 0)
-               return(n);
-       (*fn)(arg);
-       return(0);
+	sigjmp_buf buf;
+	int n, enable;
+
+	*jmp_ptr = &buf;
+	n = UML_SIGSETJMP(&buf, enable);
+	if(n != 0)
+		return(n);
+	(*fn)(arg);
+	return(0);
 }
-
-/*
- * Overrides for Emacs so that we follow Linus's tabbing style.
- * Emacs will notice this stuff at the end of the file and automatically
- * adjust the settings for this buffer only.  This must remain at the end
- * of the file.
- * ---------------------------------------------------------------------------
- * Local variables:
- * c-file-style: "linux"
- * End:
- */

arch/um/os-Linux/signal.c

@@ -20,23 +20,58 @@
 #include "mode.h"
 #include "os.h"
 
+/* These are the asynchronous signals.  SIGVTALRM and SIGARLM are handled
+ * together under SIGVTALRM_BIT.  SIGPROF is excluded because we want to
+ * be able to profile all of UML, not just the non-critical sections.  If
+ * profiling is not thread-safe, then that is not my problem.  We can disable
+ * profiling when SMP is enabled in that case.
+ */
+#define SIGIO_BIT 0
+#define SIGIO_MASK (1 << SIGIO_BIT)
+
+#define SIGVTALRM_BIT 1
+#define SIGVTALRM_MASK (1 << SIGVTALRM_BIT)
+
+#define SIGALRM_BIT 2
+#define SIGALRM_MASK (1 << SIGALRM_BIT)
+
+static int signals_enabled = 1;
+static int pending = 0;
+
 void sig_handler(ARCH_SIGHDLR_PARAM)
 {
 	struct sigcontext *sc;
+	int enabled;
+
+	/* Must be the first thing that this handler does - x86_64 stores
+	 * the sigcontext in %rdx, and we need to save it before it has a
+	 * chance to get trashed.
+	 */
 
 	ARCH_GET_SIGCONTEXT(sc, sig);
+
+	enabled = signals_enabled;
+	if(!enabled && (sig == SIGIO)){
+		pending |= SIGIO_MASK;
+		return;
+	}
+
+	block_signals();
+
 	CHOOSE_MODE_PROC(sig_handler_common_tt, sig_handler_common_skas,
 			 sig, sc);
+
+	set_signals(enabled);
 }
 
 extern int timer_irq_inited;
 
-void alarm_handler(ARCH_SIGHDLR_PARAM)
+static void real_alarm_handler(int sig, struct sigcontext *sc)
 {
-	struct sigcontext *sc;
-
-	ARCH_GET_SIGCONTEXT(sc, sig);
-	if(!timer_irq_inited) return;
+	if(!timer_irq_inited){
+		signals_enabled = 1;
+		return;
+	}
 
 	if(sig == SIGALRM)
 		switch_timers(0);
@@ -46,17 +81,52 @@ void alarm_handler(ARCH_SIGHDLR_PARAM)
 
 	if(sig == SIGALRM)
 		switch_timers(1);
+
+}
+
+void alarm_handler(ARCH_SIGHDLR_PARAM)
+{
+	struct sigcontext *sc;
+	int enabled;
+
+	ARCH_GET_SIGCONTEXT(sc, sig);
+
+	enabled = signals_enabled;
+	if(!signals_enabled){
+		if(sig == SIGVTALRM)
+			pending |= SIGVTALRM_MASK;
+		else pending |= SIGALRM_MASK;
+
+		return;
+	}
+
+	block_signals();
+
+	real_alarm_handler(sig, sc);
+	set_signals(enabled);
 }
 
 extern void do_boot_timer_handler(struct sigcontext * sc);
 
 void boot_timer_handler(ARCH_SIGHDLR_PARAM)
 {
 	struct sigcontext *sc;
+	int enabled;
 
 	ARCH_GET_SIGCONTEXT(sc, sig);
 
+	enabled = signals_enabled;
+	if(!enabled){
+		if(sig == SIGVTALRM)
+			pending |= SIGVTALRM_MASK;
+		else pending |= SIGALRM_MASK;
+		return;
+	}
+
+	block_signals();
+
 	do_boot_timer_handler(sc);
+	set_signals(enabled);
 }
 
 void set_sigstack(void *sig_stack, int size)
@@ -83,6 +153,7 @@ void set_handler(int sig, void (*handler)(int), int flags, ...)
 {
 	struct sigaction action;
 	va_list ap;
+	sigset_t sig_mask;
 	int mask;
 
 	va_start(ap, flags);
@@ -95,7 +166,12 @@ void set_handler(int sig, void (*handler)(int), int flags, ...)
 	action.sa_flags = flags;
 	action.sa_restorer = NULL;
 	if(sigaction(sig, &action, NULL) < 0)
-		panic("sigaction failed");
+		panic("sigaction failed - errno = %d\n", errno);
+
+	sigemptyset(&sig_mask);
+	sigaddset(&sig_mask, sig);
+	if(sigprocmask(SIG_UNBLOCK, &sig_mask, NULL) < 0)
+		panic("sigprocmask failed - errno = %d\n", errno);
 }
 
 int change_sig(int signal, int on)
@@ -108,91 +184,74 @@ int change_sig(int signal, int on)
 	return(!sigismember(&old, signal));
 }
 
-/* Both here and in set/get_signal we don't touch SIGPROF, because we must not
- * disable profiling; it's safe because the profiling code does not interact
- * with the kernel code at all.*/
-
-static void change_signals(int type)
-{
-	sigset_t mask;
-
-	sigemptyset(&mask);
-	sigaddset(&mask, SIGVTALRM);
-	sigaddset(&mask, SIGALRM);
-	sigaddset(&mask, SIGIO);
-	if(sigprocmask(type, &mask, NULL) < 0)
-		panic("Failed to change signal mask - errno = %d", errno);
-}
-
 void block_signals(void)
 {
-	change_signals(SIG_BLOCK);
+	signals_enabled = 0;
 }
 
 void unblock_signals(void)
 {
-	change_signals(SIG_UNBLOCK);
-}
+	int save_pending;
 
-/* These are the asynchronous signals.  SIGVTALRM and SIGARLM are handled
- * together under SIGVTALRM_BIT.  SIGPROF is excluded because we want to
- * be able to profile all of UML, not just the non-critical sections.  If
- * profiling is not thread-safe, then that is not my problem.  We can disable
- * profiling when SMP is enabled in that case.
- */
-#define SIGIO_BIT 0
-#define SIGVTALRM_BIT 1
-
-static int enable_mask(sigset_t *mask)
-{
-	int sigs;
+	if(signals_enabled == 1)
+		return;
 
-	sigs = sigismember(mask, SIGIO) ? 0 : 1 << SIGIO_BIT;
-	sigs |= sigismember(mask, SIGVTALRM) ? 0 : 1 << SIGVTALRM_BIT;
-	sigs |= sigismember(mask, SIGALRM) ? 0 : 1 << SIGVTALRM_BIT;
-	return(sigs);
+	/* We loop because the IRQ handler returns with interrupts off.  So,
+	 * interrupts may have arrived and we need to re-enable them and
+	 * recheck pending.
+	 */
+	while(1){
+		/* Save and reset save_pending after enabling signals.  This
+		 * way, pending won't be changed while we're reading it.
+		 */
+		signals_enabled = 1;
+
+		save_pending = pending;
+		if(save_pending == 0)
+			return;
+
+		pending = 0;
+
+		/* We have pending interrupts, so disable signals, as the
+		 * handlers expect them off when they are called.  They will
+		 * be enabled again above.
+		 */
+
+		signals_enabled = 0;
+
+		/* Deal with SIGIO first because the alarm handler might
+		 * schedule, leaving the pending SIGIO stranded until we come
+		 * back here.
+		 */
+		if(save_pending & SIGIO_MASK)
+			CHOOSE_MODE_PROC(sig_handler_common_tt,
+					 sig_handler_common_skas, SIGIO, NULL);
+
+		if(save_pending & SIGALRM_MASK)
+			real_alarm_handler(SIGALRM, NULL);
+
+		if(save_pending & SIGVTALRM_MASK)
+			real_alarm_handler(SIGVTALRM, NULL);
+	}
 }
 
 int get_signals(void)
 {
-	sigset_t mask;
-
-	if(sigprocmask(SIG_SETMASK, NULL, &mask) < 0)
-		panic("Failed to get signal mask");
-	return(enable_mask(&mask));
+	return signals_enabled;
 }
 
 int set_signals(int enable)
 {
-	sigset_t mask;
 	int ret;
+	if(signals_enabled == enable)
+		return enable;
 
-	sigemptyset(&mask);
-	if(enable & (1 << SIGIO_BIT))
-		sigaddset(&mask, SIGIO);
-	if(enable & (1 << SIGVTALRM_BIT)){
-		sigaddset(&mask, SIGVTALRM);
-		sigaddset(&mask, SIGALRM);
-	}
-
-	/* This is safe - sigprocmask is guaranteed to copy locally the
-	 * value of new_set, do his work and then, at the end, write to
-	 * old_set.
-	 */
-	if(sigprocmask(SIG_UNBLOCK, &mask, &mask) < 0)
-		panic("Failed to enable signals");
-	ret = enable_mask(&mask);
-	sigemptyset(&mask);
-	if((enable & (1 << SIGIO_BIT)) == 0)
-		sigaddset(&mask, SIGIO);
-	if((enable & (1 << SIGVTALRM_BIT)) == 0){
-		sigaddset(&mask, SIGVTALRM);
-		sigaddset(&mask, SIGALRM);
-	}
-	if(sigprocmask(SIG_BLOCK, &mask, NULL) < 0)
-		panic("Failed to block signals");
+	ret = signals_enabled;
+	if(enable)
+		unblock_signals();
+	else block_signals();
 
-	return(ret);
+	return ret;
 }
 
 void os_usr1_signal(int on)